1. FIELD OF THE INVENTION
This invention relates to a harmonic eliminating apparatus for removing and/or absorbing harmonics of AC bus voltage generated by a thyristor converter or the like.
2. DESCRIPTION OF PRIOR ART
Recently, a thyristor converter is has been widely used for controlling power supplied to a load by changing the firing angle thereof. In view of the fact that the firing angle is controlled, the current on the power supply side of the thyristor converter is greatly distorted, which causes an increase of harmonic components thereby not only reducing the power factor of the thyristor but also inducing noise interference with adjacent communication lines. A known measure for obviating this inconvenience consists in providing a filter with a static capacitor. Such a filter absorbs reactive power together with harmonics. This poses the problem of how the reactive power is to be disposed of in the case of an AC power supply having small capacity. In other words, when the load on the AC power supply changes to become small, a phase-advanced current flows in the AC power supply. As a result, the source voltage becomes excessive, which often damages the devices connected to the AC power supply.
In order to obviate this problem, a measure conventionally taken is described, for example, in the literature mentioned below.
Ivor R. Smith: "Reactive-Current Compensation by Switched Capacitors", IEEE Transactions on Industrial Electronics and Instrumentation, Vol. IECI-22, No. 1, Feb. 1975, pp. 75-78
The described measure is such that a capacitor is connected in series with a thyristor so that the capacitance of the capacitor is changed by on-off control of the thyristor. Such a measure prevents the source voltage from becoming excessive under a light load. Nevertheless, the resonance frequency of the filter also undergoes a change with a change in the capacity of the capacitor, with a result that the effect of harmonics elimination is reduced. Therefore, satisfactory elimination of harmonics is impossible.